ACRYLIC FIBER STABILIZATION CATALYZED BY Co(II) AND Ce(III) CATIONS

ABSTRACT

A process is provided wherein the thermal stabilization of an acrylic fibrous material is accelerated by heating in an oxygencontaining atmosphere in the presence of a catalytic quantity of Co(II) and Ce(III) metallic cations which have been found capable of promoting the oxidative cross-linking of adjoining polymer molecules. The resulting stabilized fibrous materials are nonburning when subjected to an ordinary match flame, and may be utilized as fire resistant textile fibers, or optionally converted to a carbonized fibrous material by heating in an inert atmosphere at a more highly elevated temperature.

nited States Patent [151 3,656,2 [451 Apr. 18,1972

Riggs 154] ACRYLIC FIBER STABILIZATION CATALYZED BY CO(II) AND CE(III) CATIONS [72] Inventor: John Perry Riggs, Berkeley Heights, NJ. [73] Assignee: Celanese Corporation, New York, NY. [22] Filed: Mar. 9, 1970 [21] App1.No.: 17,965

[52] U.S. Cl ..8/115.5,23/209.1, 23/2094 Y [51] Int. Cl. ..C01b 31/07' [58] Field ofSearch ..23/209.1, 209.2, 209.4; 8/115.5, 115.6; 117/46, 138; 252/506, 8.1;106/15 [56] References Cited UNITED STATES PATENTS 3,242,000 3/1966 Lynch ..117/46 3,416,874 12/1968 Robin ..117/46 X 3,529,934 9/1970 Shindo ..23/209.1 2,799,915 7/1957 Barnett etal. .....8/115.5 3,412,062 1l/l968 Johnson et al. ..260/37 Primary Examiner-Edward J. Meros- Attorney-Thomas J. Morgan, Charles B. Barris and Kenneth E. Macklin 57 ABSTRACT A process is provided wherein the thermal stabilization of an acrylic fibrous material is accelerated by heating in an oxygencontaining atmosphere in the presence of a catalytic quantity of Co(ll) and Ce(lll) metallic cations which have been found capable of promoting the oxidative cross-linking of adjoining polymer molecules. The resulting stabilized fibrous materials are non-buming when subjected to an ordinary match flame, and may be utilized as fire resistant textile fibers, or optionally converted to a carbonized fibrous material by heating in an inert atmosphere at a more highly elevated temperature.

16 Claims, No Drawings ACRYLIC FIBER STABILIZATION CATALYZED BY C(II) AND CE(III) CATIONS BACKGROUND OF THE INVENTION 3,285,696 to Tsunoda disclose processes for the conversion of fibers of acrylonitrile homopolymers or copolymers to a heat resistant form. The stabilization of fibers of acrylonitrile homopolymers and copolymers in an oxygen-containing atmosphere involves (1) an oxidative cross-linking reaction of adjoining molecules as well as (2) a cyclization reactionof pendant nitrile groups. It is generally recognized that the rate at which the stabilization reaction takes placeincreases with i the temperature of the oxygen-containing atmosphere. However, the stabilization reaction must by necessity be conducted at relatively low temperatures i.e. below about 300 C.), since the cyclization reaction is exothermic in nature and must be controlled if the original fibrous configuration of the material undergoing stabilization is'to'be preserved. Accordingly the stabilization reaction tends to be time consuming, and economically demanding because of low productivity necessitated by the excessive time requirements. Prior processes proposed to shorten the period required by the stabilization reaction include that disclosed in U.S. Pat. No. 3,4l6,874.

While stabilized acrylic fibrous materials may be used directly in applications where a non-burning fiber is required, demands for the same have been increasingly presented by manufacturers of carbonized fibrous materials. Carbonized fibrous materials are commonly formed by heating a stabilized acrylic fibrous material in an inert atmosphere, such as nitrogen or argon, at a more highly elevated temperature. During the carbonization reaction elements present in the fiber such as nitrogen, oxygen, and hydrogen are substantially expelled. Accordingly, the term carbonized" fibrous material as used in the art commonly designates a fibrous material consisting of at least about 90 per cent carbon by weight, and generally at least about 95 per cent carbon by weight. Depending upon the conditions under which the carbonized fibrous material is processed, it may or may not contain graphitic carbon as determined by the characteristic x-ray diffraction pattern of graphite. See, for instance, commonly assigned U.S. Ser. No. 777,275, filed Nov. 20, 1968, of Charles M. Clarke for a preferred procedure for forming carbonized and graphitized fibrous materials from a stabilized acrylic fibrous material. An alternate approach for forming carbonized fibers directly from acrylic fibers while coated with a refractory barrier coating is proposed in U.S. Pat. Nos. 3,242,000 and 3,281,261 to Lynch.

it is an object of the invention to provide an improved process for enhancing the thermal stability of an acrylic fibrous material.

It is an object of the invention to provide a process wherein the oxidative cross-linking reaction in the stabilization of an acrylic fibrous material is accelerated.

it is an object of the invention to provide a process for producing a stabilized acylic fibrous material wherein the oxidative cross-linking reaction yields a highly uniform stabilized structure.

It is an object of the invention to provide a stabilized acrylic fibrous material which is highly amenable for utilization as a precursor in the formation of amorphous carbon or graphitic carbon fibrous materials.

It is an object of the invention to provide a process for the stabilization of acrylic fibrous materials which is readily adaptable to fibers of varying deniers.

These and other objects, as well as the scope, nature and utilization of the invention will be apparent from the following detailed description and appended claims.

LII

SUMMARY OF THE INVENTION It has been found that an improved process for the stabilization of an acrylic fibrous material selected from the group consisting of an acrylonitrile homopolymer and acrylonitrile copolymers containing at least about 85 mol per cent of acrylonitrile units and up to about mol per cent of one or more monovinyl units copolymerized-therewith comprises:

a. providing said acrylic fibrous material in intimate association with a catalytic quantity up toabout 0.5 per cent by weight based upon the weight of the acrylic fibrous material of metallic cations selected from the group consisting of Co(ll) and Ce(lll),and

b. heating said acrylic fibrous material in an oxygen-containing atmosphere at a temperature of about 200 to 290 C. while in intimate association with said metallic. cations until a stabilized product is formed which retains its original fibrous configuration essentially intact and which is non-burning when subjected to an ordinary match flame.

The stabilized products formed in accordance with the present process commonly exhibit a bound oxygen content of at least about 7 per cent by weight, and a carbon content of about to 'per cent by weight.

DESCRIPTION OE PREFERRED EMBODIMENTS The acrylic fibrous materials undergoing stabilization in the present process may be formed by conventional solution spinning techniques (i.e. may be dry spun or wet spun), and are commonly drawn to increase their orientation. As is known in the art, dry spinning is commonly conducted by dissolving the polymer in an appropriate solvent, such as N,N- dimethyl formamide or N,N-dimethyl acetamide, and passing the solution through an opening of predetermined shape into an evaporative atmosphere (e.g. nitrogen) in which much of the solvent is evaporated. Wet spinning is commonly conducted by passing a solution of the polymer through an opening of predetermined shape into an aqueous coagulation bath.

The acrylic polymer utilized as the starting material is formed primarily of recurring acrylonitrile units. For instance, the acrylic polymer should generally contain not less than about mol per cent of acrylonitrile units and not more than about 15 mol per cent of units derived from a monovinyl compound which is copolymerizable with acrylonitrile such as styrene, methyl acrylate, methyl methacrylate, vinyl acetate, vinyl chloride, vinylidene chloride, vinyl pyridine, and the like, or a plurality of such monomers.

The preferred acrylic fibrous material is an acrylonitrile homopolymer. Preferred acrylonitrile copolymers contain at least about mol per cent of acrylonitrile units and up to about 5 mol per cent of one or more monovinyl units copolymerized therewith.

The acrylic fibrous materials are provided as continuous lengths and may be in a variety of physical configurations. For instance, the acrylic fibrous materials may be present in the form of continuous lengths of multifilament yarns, tows, tapes,

strands, cables, or similar fibrous assemblages.

When the starting material is a continuous multifilament yarn, a twist may be imparted to the same to improve the handling characteristics. For instance, a twist of about 0.l to 5 tpi,

'and preferably about 0.3 to 1.0 tpi may be utilized. Also, a

false twist may be used instead of or in addition to a real twist. Alternatively, one may select bundles of fibrous material I which possess essentially no twist.

The starting material may be drawn in accordance with conventional techniques in order to improve its orientation. For instance, the starting material may be drawn by stretching while in contact with a hot shoe at a temperature of about to C. Additional representative drawing techniques are disclosed in U.S. Pat. Nos. 2,455,173; 2,948,581; and 3,122,412. It is recommended that the acrylic fibrous materials selected for use in the process be drawn to a single filament tenacity of at least about 3 grams per denier. If desired, however, the starting material may be more highly oriented, e.g. drawn up to a single filament tenacity of about 7.5 to 8 grams per denier, or more.

Prior to heating the acrylic fibrous material in an oxygencontaining atmosphere to accomplish the desired stabilization (as described hereafter), the fibrous material is provided in intimate association with a catalytic quantity up to about 0.5 per cent by weight based upon the weight of the fibrous material of Co(ll) or Ce(lll) metallic cations, or mixtures of the same.

As will be apparent to those skilled in the art, the source of the metallic cations may be varied. Illustrative examples of compounds capable of yielding the desired Co(II) metallic cations are as follows: cobaltous chloride [CoCl '6 H O], cobaltous nitrate [Co(NO "l-I O], cobaltous carbonate [2CoCO '3 Co(OH) cobaltous sulfate [CoSO 'H O], cobaltous bromide [CoBr '6 H and cobaltous acetate [Co(C H O 4H Oa) The preferred source of Co (II) metallic cations is cobaltous chloride [CoCl '6 H O].

Illustrative examples of compounds capable of yielding the desired Ce(lll) metallic cations are as follows: cerous chloride [CeClJ H O], cerous nitrate [Ce(NO -6H O], and cerous sulfate [Ce(SO '8I-I O]. The preferred source of Ce(lll) metallic cations is cerous chloride [CeCl '7I-I O].

The intimate association of the acrylic fibrous material and the metallic cations is preferably accomplished by contacting the fibrous material with a solution containing the Co(II) or Ce(III) cations dissolved therein, and subsequently drying the fibrous material whereby the solvent of the solution in contact with the acrylic fibrous material is substantially expelled.

The solution from which the metallic cations are applied is preferably aqueous in nature. Solvents other than water, which are capable of dissolving the source compounds for the Co(ll) and Ce(lll) cations, may likewise be selected provided the solvents do not adversely influence the properties of the acrylic fibrous material. In a preferred embodiment of the invention compounds capable ofyielding the Co(ll) and Ce(lll) metallic cations upon dissolution are provided as aqueous solutions ofabout 0.0001 to 10 molarity.

The temperature of the solution containing the metallic cations while contacted with the acrylic fibrous material may be from below ambient up to below that temperature at which the properties of the acrylic fibrous material are adversely influenced. In a preferred embodiment of the invention the acrylic fibrous material is contacted with an aqueous solution containing the Co(ll) or Ce(lll) metallic cations which is at a temperature of about 10 to 95 C. In a particularly preferred embodiment of the invention the solution is conveniently providcd at ambient temperature (i.e. at about C.).

The technique employed to make contact between the acrylic fibrous material and the solution ofthe metallic cations may be varied as will be apparent to those skilled in the art. In a preferred embodiment of the invention contact is made by immersing the acrylic fibrous material in a vessel containing a solution of the metallic cations. Alternatively, contact may be made by spraying the acrylic fibrous material with a solution of the metallic cations. The duration of the period of contact between the acrylic fibrous material and the solution is not critical provided the requisite catalytic quantity ofthe metallic cations up to about 0.5 per cent by weight based upon the weight of the acrylic fibrous material is ultimately provided in intimate association with the fibrous material upon drying. For

instance, the quantity of the metallic cations, provided in intimate association with the acrylic fibrous material may be from about 0.000l to 0.5 per cent by weight based upon the weight of the dried acrylic fibrous material. The duration of the period of contact will be influenced to some degree by the metallic cations concentration of the solution, the temperature of the solution, the degree of compaction of the acrylic fibrous material undergoing treatment, and the denier of the acrylic fibrous material. Contact times of about 5 seconds to 48 hours, or more, may be selected. A greater diffusion ofthe metallic cations into the acrylic fibrous material occurs with longer residence times. A continuous length of the acrylic fibrous material may be wound upon a support and statically contacted with the solution containing the metallic cations. Alternatively, a continuous length of the acrylic fibrous material may be continuously passed in the direction of its length through a vessel or zone in which the solution is provided.

The drying of the fibrous material following contact with the solution containing the metallic cations may be conducted in any convenient manner. For instance, the fibrous material may be simply exposed to ambient conditions until the solvent adhering thereto is substantially evaporated. The drying step can, of course, be expedited by exposure to a circulating gaseous atmosphere provided at an elevated-temperature, as will be apparent to those skilled in the art. If desired, the drying may be conveniently conducted in the same zone in which the stabilization reaction is carried out, as described hereafter. Upon removal of the solvent a thin catalytic residue of Co(ll) and Ce(lll) metallic cations is deposited upon the fiber surface. Also, internal diffusion of the metallic cations into the fibrous material is achieved.

The acrylic fibrous material while in intimate association with the metallic cations is next exposed to an oxygen-containing atmosphere at a temperature of about 200 to 290 C. until a stabilized fibrous product is formed. In a preferred embodiment of the invention the oxygen-containing atmosphere is air. Preferred temperatures for the oxygen-containing atmosphere are about 220 to 260 C., and most preferably about 240 to 250 C.

For best results uniform contact during the stabilization reaction with molecular oxygen throughout all portions of the acrylic fibrous material is encouraged. Such uniform reaction conditions can best be accomplished by limiting the mass of fibrous material at any one location so that heat dissipation from within the interior of the fibrous material is not unduly impaired, and free access to molecular oxygen is provided. For instance, the acrylic fibrous material may be placed in the oxygen-containing atmosphere while wound upon a support to a limited thickness. In a preferred embodiment of the invention the acrylic fibrous material while in intimate association with the metallic cations is continuously passed in the direction of its length through the heated oxygen-containing atmosphere. For instance, a continuous length of the acrylic fibrous material may be passed through a circulating oven or the tube of a muffiefurnace. The speed of passage through the heated oxygen-containing atmosphere will be determined by the size of the heating zone and the desired residence time.

The period of time required to complete the stabilization reaction within the oxygen-containing atmosphere is generally inversely related to the temperature of the atmosphere, and is also influenced by the denier of the acrylic fibrous material undergoing treatment. Treatment times in the oxygen-containing atmosphere accordingly commonly range from about 30 minutes to hours. Regardless of the stabilization temperature selected within the range of about 200 to 290 C. the intimate presence of the Co(ll) or Ce(IIl) metallic cations as described results in an accelerated oxidative cross-linking reaction for a given temperature.

The stabilized acrylic fibrous materials formed in accordance with the present process are black in appearance, retain essentially the same fibrous configuration as the starting material, are non-burning when subjected to an ordinary match flame, commonly have a bound oxygen content of at least 7 per cent by weight as determined by the Unterzaucher analysis, and commonly contain from about 50 to 65 per cent carbon by weight.

The theory whereby the Co(ll) and Ce(III) metallic cations serve to catalyze the stabilization reaction in the present process is considered complex and incapable of simple explanation. It is believed, however, that the Co(ll) and Ce(lll) cations have the ability to attract a greater quantity of molecular oxygen to the acrylic fiber than would otherwise occur in the absence of such cations, and that the oxygen is bound by the cations in close proximity to the fiber. Electron exchange and the oxidative cross-linking reaction is accordingly accelerated.

Since the oxidative-cross linking reaction is accelerated in the present process, one optionally may elect to carry out the stabilization reaction at a less severe temperature than heretofore commonly utilized. Under milder temperature conditions a more uniform stabilized fiber may be achieved in the absence of undue chain degradation.

The stabilized fibrous material resulting from the stabilization treatment of the present invention is suitable for use in applications where a fire resistance fibrous material is required. For instance, non-burning fabrics may be formed from the same. As previously indicated, the stabilized acrylic fibrous materials are particularly suited for use as intermediates in the production of carbonized fibrous materials. Such amorphous carbon or graphitic carbon fibrous products may be incorporated in a binder or matrix and serve as a reinforcing medium. The carbon fiber component may accordingly serve as a light weight load bearing component in high performance composite structures which find particular utility in the aerospace industry.

The acceleration of the stabilization reaction by the intimate presence of Co(ll) and Ce(lll) metallic cations is demonstrated by the following data. A continuous length of a 800 fil dry spun acrylonitrile homopolymer continuous filament yarn having a total denier of 1200 was selected as the starting material. The yarn was dry spun from a solution of the same in N,N-dimethyl formamide solvent into an evaporative atmosphere of nitrogen. The fibrous material was dry spun as a 40 fil bundle, and plied to form the 800 fil yarn which ex hibited a twist of about 0.5 tpi. The yarn was next drawn at a draw ratio of about 5:1 to a single filament tenacity of about 4 grams per denier by stretching while passing over a hot shoe at a temperature of about 160 C. for a residence time of about 0.5 second. Four segments of the yarn were wound on four different porous bobbins and were given the following treatments:

1. Sample A was designated the control, and was placed in a circulating air oven maintained at 220 C. for 90 minutes. At the end of this period of time a bound oxygen content within the fibrous material of 0.78 per cent by weight as determined by the Unterzaucher analysis was observed.

2. Sample B was immersed in a vessel containing a 0.042 Molar aqueous solution of cobaltous chloride [CoCl 6H O] provided at 25 C. for 16 hours, was removed from the vessel, was allowed to dry at ambient conditions, and was placed in a circulating air oven at 220 C. for 90 minutes. At the end of this period of time a bound x e .sqnt t t Z-iLPstEsn by e tta determined by the Unterzaucher analysis was observed.

. Sample C was immersed in a vessel containing a 0.17 Molar aqueous solution of cobaltous chloride [CoCl 6H O] provided at 25 C. for 16 hours, was removed from the vessel, was allowed to dry at ambient condi-. tions, and was placed in a circulating air oven at 220 C. for 90 minutes. At the end of this period of time a bound oxygen content of 3.52 percent by weight as e m ned by theiln ziqsb QBFllL lLWQQWEEl EQ- Sample D was immersed in a vessel containing a 0.07 Molar aqueous solution of cerous chloride [CeCl -7l-l O] provided at 25 C. for 16 hours, was removed from the vessel, was allowed to dry at ambient conditions, and was placed in a circulating air oven at 220 C. for 90 minutes. At the end of this period of time a bound oxygen content of 1.56 per cent by weight as determined by the Unterzaucher analysis was observed.

The following examples are given as specific illustrations of the invention. It should be understood, however, that the invention is not limited to the specific details set forth in the examples.

EXAMPLE I This example illustrates the stabilization of an acrylic fibrous material in the presence of Co(ll) metallic cations in accordance with the present invention.

A continuous length of a 1600 fil dry spun acrylonitrile homopolymer continuous filament yam having a total denier of 1920 is selected as the starting material. The yarn is initially dry spun from a solution of the same in N,N-dimethyl formamide solvent into an evaporative atmosphere of nitrogen. The yarn is spun as a 40 fil bundle, and plied to form the 1600 fil yarn which exhibits a twist of about 0.5 tpi. The yarn is next drawn at a draw ratio of about 5:1 to a single filament tenacity of about 4 grams per denier by stretching while passing over a hot shoe at a temperature of about 160 C. for a resistance time of about 0.5 second.

A solution containing Co(ll) metallic cations is formed by dissolving cobaltous chloride [CoCl 2'6H O] in water to form a 0.2 Molar solution of the compound. The acrylonitrile homopolymer yarn while wound upon a perforated bobbin is immersed for 10 hours in a vessel containing the aqueous solution provided at 25 C., is removed from the vessel, is allowed to dry at ambient conditions (i.e. 25 C.) for l hour, and is placed in a circulating air oven at 245 C. for 3 hours.

The resulting stabilized yarn retains its original fibrous configuration essentially intact, and is non-burning when subjected to an ordinary match flame. The bound oxygen content of the yarn as determined by the Unterzaucher analysis is in excess of 7 per cent by weight, and the carbon content is less than 65 per cent by weight. The product is capable of undergoing carbonization and graphitization in accordance with the teachings of commonly assigned U.S. Ser. No. 777,275, filed Nov. 20,1968, of Charles M. Clarke which is herein incorporated by reference.

EXAMPLE ll Example I is repeated with the exception that the stabilization reaction is catalyzed by Ce(lll) metallic cations rather than Co(ll) metallic cations. More specifically, a solution containing Ce(llI) metallic cations is formed by dissolving cerous chloride [CeCl '7H O] in water to form a 0.2 Molar solution of the compound. The yarn is immersed in this solution as described in Example I. Substantially similar results are achieved.

Although the invention has been described with preferred embodiments, it is to be understood that variations and modifications may be resorted to as will be apparent to those skilled in the art. Such variations and modifications are to be considered within the purview and scope of the claims appended hereto.

I claim I. An improved process for the stabilization of an acrylic fibrous material selected from the group consisting of an acrylonitrile homopolymer and acrylonitrile copolymers containing at least about mol per cent of acrylonitrile units and up to about 15 mol per cent of one or more monovinyl units copolymerized therewith comprising:

a. providing said acrylic fibrous material in intimate association with a catalytic quantity up to about 0.5 per cent by weight based upon the weight of the acrylic fibrous material of metallic cations selected from the group consisting of Co(ll) and Ce)IIl), and

. heating said acrylic fibrous material in an oxygen-containing atmosphere at a temperature of about 200 to 290 C. while in intimate association with said metallic cations until a stabilized product is formed which retains its original fibrous configuration essentially intact, is nonbuming when subjected to an ordinary match flame, contains a bound oxygen content of at least about 7 per cent by weight, and a carbon content of about 50 to 65 per cent by weight, with said metallic cations serving to accelerate the oxidative portion of said stabilization reaction.

2. A process according to claim 1 wherein said acrylic fibrous material is an acrylonitrile homopolymer.

3. A process according to claim 1 wherein said acrylic fibrous material is an acrylonitrile copolymer containing at least about 95 mol per cent of acrylonitrile units and up to about mol per cent of one or more monovinyl units copolymerized therewith.

4. A process according to claim 1 wherein said acrylic fibrous material has been drawn to a single filament tenacity of at least about 3 grams per denier.

5. A process according to claim 1 wherein said metallic cations in intimate association with said acrylic fibrous material are provided in a quantity of about 0.0001 to 0.5 per cent by weight based upon the weight of said acrylic fibrous material.

6. A process according to claim 1 wherein said metallic cations are Co(ll).

7. A process according to claim 1 wherein said metallic cations are Ce(lll).

8. An improved process for enhancing the thermal stability of an acrylic fibrous material selected from the group consisting of an acrylonitrile homopolymer and acrylonitrile copolymers containing at least about 85 mol per cent of acrylonitrile units and up to about mol per cent of one or more monovinyl units copolymerized therewith comprising:

a. contacting said acrylic fibrous material with a solution containing metallic cations selected from the group consisting of Co(ll) and Ce(lll),

b. drying said fibrous material whereby the solvent of said solution in contact with said acrylic fibrous material is substantially expelled and said fibrous material is provided in intimate association with a catalytic quantity up to about 0.5 per cent by weight based upon the weight of the acrylic fibrous material of said metallic cations, and

. heating the resulting acrylic fibrous material in an oxygen-containing atmosphere at a temperature of about 200 to 290 C. while in intimate association with said metallic cations until a stabilized product is formed which retains its original fibrous configuration essentially intact,

is non-buming when subjected to an ordinary match flame, contains a bound oxygen content of at least about 7 per cent by weight, and a carbon content of about 50 to 65 per cent by weight, with said metallic cations serving to accelerate the oxidative portion of said stabilization reaction.

9. A process according to claim 8 wherein said acrylic fibrous material is an acrylonitrile homopolymer.

10. A process according to claim 8 wherein said acrylic fibrous material is an acrylonitrile copolymer containing at least about mol per cent of acrylonitrile units and up to about 5 mol per cent of one or more monovinyl units copolymerized therewith.

11. A process according to claim 8 wherein said acrylic fibrous material has been drawn to a single filament tenacity of at least about 3 grams per denier.

12. A process according to claim 8 wherein said solution containing said metallic cations contacted with said acrylic fibrous material is an aqueous solution which is at a temperature of about 10 to 95 C.

13. A process according to claim 8 wherein said solution containing said metallic cations is an aqueous solution of cobaltous chloride.

14. A process according to claim 8 wherein said solution containing said metallic cations is an aqueous solution of cerous chloride.

15. A process according to claim 8 wherein said drying yields said fibrous material in intimate association with about 0.0001 to 0.5 per cent by weight of said metallic cations based upon the weight of said acrylic fibrous material.

16. A process according to claim 8 wherein said resulting acrylic fibrous material is heated in an oxygen-containing atmosphere at a temperature of about 240 to 250 C. 

2. A process according to claim 1 wherein said acrylic fibrous material is an acrylonitrile homopolymer.
 3. A process according to claim 1 wherein said acrylic fibrous material is an acrylonitrile copolymer containing at least about 95 mol per cent of acrylonitrile units and up to about 5 mol per cent of one or more monovinyl units copolymerized therewith.
 4. A process according to claim 1 wherein said acrylic fibrous material has been drawn to a single filament tenacity of at least abOut 3 grams per denier.
 5. A process according to claim 1 wherein said metallic cations in intimate association with said acrylic fibrous material are provided in a quantity of about 0.0001 to 0.5 per cent by weight based upon the weight of said acrylic fibrous material.
 6. A process according to claim 1 wherein said metallic cations are Co(II).
 7. A process according to claim 1 wherein said metallic cations are Ce(III).
 8. An improved process for enhancing the thermal stability of an acrylic fibrous material selected from the group consisting of an acrylonitrile homopolymer and acrylonitrile copolymers containing at least about 85 mol per cent of acrylonitrile units and up to about 15 mol per cent of one or more monovinyl units copolymerized therewith comprising: a. contacting said acrylic fibrous material with a solution containing metallic cations selected from the group consisting of Co(II) and Ce(III), b. drying said fibrous material whereby the solvent of said solution in contact with said acrylic fibrous material is substantially expelled and said fibrous material is provided in intimate association with a catalytic quantity up to about 0.5 per cent by weight based upon the weight of the acrylic fibrous material of said metallic cations, and c. heating the resulting acrylic fibrous material in an oxygen-containing atmosphere at a temperature of about 200* to 290* C. while in intimate association with said metallic cations until a stabilized product is formed which retains its original fibrous configuration essentially intact, is non-burning when subjected to an ordinary match flame, contains a bound oxygen content of at least about 7 per cent by weight, and a carbon content of about 50 to 65 per cent by weight, with said metallic cations serving to accelerate the oxidative portion of said stabilization reaction.
 9. A process according to claim 8 wherein said acrylic fibrous material is an acrylonitrile homopolymer.
 10. A process according to claim 8 wherein said acrylic fibrous material is an acrylonitrile copolymer containing at least about 95 mol per cent of acrylonitrile units and up to about 5 mol per cent of one or more monovinyl units copolymerized therewith.
 11. A process according to claim 8 wherein said acrylic fibrous material has been drawn to a single filament tenacity of at least about 3 grams per denier.
 12. A process according to claim 8 wherein said solution containing said metallic cations contacted with said acrylic fibrous material is an aqueous solution which is at a temperature of about 10* to 95* C.
 13. A process according to claim 8 wherein said solution containing said metallic cations is an aqueous solution of cobaltous chloride.
 14. A process according to claim 8 wherein said solution containing said metallic cations is an aqueous solution of cerous chloride.
 15. A process according to claim 8 wherein said drying yields said fibrous material in intimate association with about 0.0001 to 0.5 per cent by weight of said metallic cations based upon the weight of said acrylic fibrous material.
 16. A process according to claim 8 wherein said resulting acrylic fibrous material is heated in an oxygen-containing atmosphere at a temperature of about 240* to 250* C. 